Spark plug and method for making same



W. C. COLE SPARK PLUG AND-METHOD FOR MAKING SAME Filed Dec. 12, 1955 Sept. 1, 1959 [N VENTORY.

4 7// 7///, 1/: fl/w l ill g flo ATMA /VQ SPARK PLUG METHOD FOR MAKING SAME William C. Cole, Grand Blanc, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware 1 Application December 12, 1955, Serial No. 552,469

9 Claims. 01. 313-444) jThisinvention relates to spark plugs and to a method for making same. More particularly, it relates to an improved structure and method I for making such a spark plug.

More particularly, an object of the invention is the provision of improved ground electrode-to-shell joint having greater durability and reliability due to its increased corrosion resistance.

Other objects and advantages of the invention will appear more clearly from the following description of a preferred embodiment and from the drawings in which: Fig. 1; is a side view in section of a spark plug embodying. the invention;

1 Fig. 2 is a bottom view of the spark plug shown in Fig.

; and

.Fig. 3 is a view of a portion of the lower end of the spark plug shown in Fig. 1, but in larger scale.

Referring now to Fig. 1, there is shown an aircraft type spark plug having as its principal parts a center wire assembly 2, insulator 4, metal shell 6, and a ground electrode 8 which is secured to the base of the metal shell in -accordance with the present invention. In the particular embodiment shown, center wire 2 which extends through the insulator centerbore comprises a copper cored nickel electrode spindle 10 having its lower end extending below the insulator to form the center electrode 12, an electrical conductive seal 14 consisting of a mixture of glass and copper powder or the like, a metal plug 16 embedded in the glass seal, cartridge resistor element 18, threaded contact plug 20 which is suitably cemented within the centerbore and contact spring 22 biased between the contact plug 20 and the upper end of the resistor element. As is usual in aircraft spark plugs, a metal shielding barrel 24 which surrounds the upper end of insulator 4 is secured to and extends upwardly from metal shell 6 to shield against radio interference from the ignition system.

To secure the insulator 4 within the shell-shielding barrel assembly, external annular shoulders 26 and 28 are provided, lower shoulder 26 cooperating with internal shoulder 30 of the shell and upper shoulder 28 with the lower edge of shielding barrel 24, all in accordance with normal practice.

That part of the spark plug structure to which the present invention particularly relates is the joint between ground electrode 8 and shell 6. The preferred ground electrode comprises an annular portion 32 from which there extend inwardly into spaced relation with center UnitedStates Patent electrode 12, a plurality of prongs 34. Ground electrodes of this general type having an annular portion and inwardly extending prongs have become known in the art as cloverleaf ground electrodes. In the embodiment shown (see Fig. 2), the prongs are three in number and extend inwardly and then downwardly, the lowermost portion of each of the prongs being substantially parallel to the center electrode 12 and having an inner arcuate surface which is spaced from and parallel with center electrode 12 thus forming a uniform spark gap therewith.

To securethe ground electrode to the shell, annular portion 32 of the ground electrode is secured within internal annular groove 36 in the base of the spark plug shell by welding and turning operations as hereinafter described in detail. Then over the base of the spark plug shell which is exposed to the hot combination gases during operation, and over the exposed portion of the joint between the shell and the ground electrode, there is provided a corrosion resistant protective cap in the form of a layer 38 of heat and corrosion resistant metal which is bonded to the shell and to the ground electrode preferably by a brazing operation as hereinafter described. This can best be seen by reference to Fig. 3. In the preferred embodiment which is shown, the layer 38 is of nickel base alloy and takes the form of an annulus which completely covers the annular shaped base of the shell 6 and fills all the crevices between the shell. and the cloverleaf ground electrode 8, thereby completely hiding the joint and giving the base of the spark plug an unbroken appearance.

Nickel base alloys which have been found especially suitable for the protectivelayer are those containing from about to nickel plus phosphorous and/or boron. It is preferable that the alloy have a melting point of from about 1600? F. to 1900 F. I have successfully used as the protective layer various alloys, all falling within the following range: Ni 80%-95%, P 5%15%, B .5%-4%, plus Si and Mn and with or Without small amounts of Cr, Fe and/or Al.

An alloy which is illustrative and which has been found to be particularly advantageous is one containing about 85.5% nickel, 7.5% phosphorous and the balance substantially all consisting of small amounts of boron, silicon, manganese, chromium and iron. (Typical analysis: 85.46% nickel, 7.44% phosphorous, balance boron, silicon, manganese, iron and chromium.) Nickel base alloys containing larger amounts of boron instead of phosphorous, as in the above example, have been successfully used. However, the above-specified nickel base-phosphorous alloy is preferred because of its greater resistance to oxidation, particularly during application thereof by means of a brazing operation as hereinafter described.

Essential to the practice of the invention is the provision of a layer of corrosion and heat resistant metal over the otherwise exposed base of the spark plug shell and the ground electrode-to-shell joint to form a bonded capjwhich protects against the corrosive effects of hot combustion gases.

Other important features of the structure will be more fully understood from the following description of the preferred method of manufacture.

As the first step in the process of manufacture, the metal shell 6 is formed with an annular opening in the form of a counterbore at the base thereof, the lower portion of the side wall of the opening being, in effect, a downwardly extending flange indicated in Fig. 1 by broken line at 42. The diameter of this opening is such that the annular portion 32 of the ground electrode will fit. very snugly therewithin, the tolerance being kept at, a minimum in order that there be maximum heat transfer from the electrode to the shell. With the shell. thus formed, the annular portion 32 of the ground electrode flux) plus sufficient water to form a paste.

is positioned within the opening and then subjected to a resistance welding operation to thereby weld the upper surface of annular portion 32 to the shoulder 44 which forms the upper wall of the shell opening. In order to obtain better heatconductivity, it is preferable that this resistance weld be continuous around the entire surface of the shoulder 44 thereby covering a maximum amount 'base of the spark plug shell and in the joint between the edge of the inwardly turned flange and the ground electrode. Any suitable flux may be used for application of the nickel base braze material; we prefer one consisting of a mixture of borax, boric acid and alkaline metal fluoride, the braze material-flux mixture being in the form of a paste or thick liquid which may be conveniently applied. Such a braze material is presently available commercially from the Wall-Colmony Corporation of Detroit, Michigan, as Nicrobraz paste and contains about 62 parts by Weight water insoluble material (nickel base-phosphorous alloy) and about 38 parts by weight soluble material (boric acid, borax-alkaline earth metal If desired, the braze-flux mixture may take a solid form to permit placement of a ring of the mixture over the base of the spark plug.

After the layer of braze-flux mixture is applied, it is heated in a suitable furnace or by an induction heating operation to brazing temperature to thereby fuse and bond a continuous or unbroken layer of the nickel base alloy to the base of the shell and to the ground electrode 8 as shown by Fig. 3. During this brazing operation, the alloy 38 under the action of the flux flows down into any crevices between the shell and the ground electrode and thereby eliminates air gaps which would be detrimental to heat transfer. The alloy forms a smooth layer giving the base of the spark plug an unbroken appearance, as shown in Figs. 2 and 3.

While the preferred alloy of nickel plus phosphorous is relatively resistant to oxidation, it is preferable to use a protective atmosphere of reducing or inert gas during the brazing operation. Nitrogen serves well as such a protective gas blanket.

After the ground electrode is thus secured, a thread is cut in the spark plug shell adjacent its base in order to provide means for securing the spark plug in the combustion chamber wall of an internal combustion engine. Then the insulator core-center wire assembly along with the shielding barrel 24 and sealing gaskets 48 and 50 are positioned within the shell, all as indicated in Fig. 1 and secured therewithin by turning inwardly the flange 52 provided at the top of the spark plug shell. Additionally, the shell may be silver brazed to the shielding barrel, as is indicated in Fig. 1. To make the assembly gas-tight, the usual Cico-Weld operation may be used, this involving the application of pressure to the top and bottom of the spark plug shell while the thin section 54 is resistance heated to its softening point, thereby collapsing the shell and firmly seating insulator shoulders 26 and 28.

During this assembly operation, it is required that the insulator core-center wire assembly be maintained in a centralized position within the shell. For this purpose, it is advantageous to utilize a centralizer sleeve, made of metal or other suitable material, between the ground electrode 8 and center electrode 12. Such a sleeve, the wall thickness of which is somewhat greater than the size of the spark gap desired, may be inserted into the ground electrode so that center electrode 12 extends into the bore of the sleeve when the insulator core-center wire assembly is inserted into the shell. Then, after the entire assembly operation, including the Cico-Weld operation, is completed, the centralizer sleeve is removed.

2,902,617 g I I With the assembly thus complete, pressure is applied to the prongs 34 of the ground electrode in order to bend them inwardly toward center. electrode 12 until the proper spark gap is attained. To accomplish this latter operation, a generally cylindrical shaped metal gauge having three arcuate shaped leaves, each one of which fits be tween one of the prongs 34 and center electrode 12, may be inserted over the center electrode, pressure thereafter being applied to the prongs until they abut the leaves of the gauge. In order to attain maximum parallelism in the spark gaps, it is preferable to apply the pressure closely adjacent the base of the prongs near the protective layer of nickel base alloy. When pressure is applied at this point, most of the bending takes place adjacent the annular portion of the ground electrode and thus maximum parallelism and uniformity in spark gap size is attained.

The protective layer of corrosion resistant metal over the base of the spark plug and shell-to-electrode joint has been found to be highly advantageous chiefly because of the great increase in useful spark plug life which it provides. For example, spark plugs made in accordance with old practice and not utilizing the protective metal layer underwent considerable corrosion even during the first 250 hours of use, whereas spark plugs made in accordance with the present invention are practically free of corrosion after 500 hours of use.

It is to be understood that while the invention has been described chiefly by reference to a particular embodiment thereof, it is not so limited since various changes and modifications may be made, all within the full and intended scope of the claims which follow.

I claim:

1. In a spark plug, a generally tubular metal shell having an annular base, a metal ground electrode having a portion thereof welded within a groove in the base of said shell thereby forming a permanent joint between said electrode and said base, and a thin layer of heat and corrosion resistant nickel base alloy bonded to said base and to said electrode, said layer forming a protective coating on said base and over the joint between said base and said electrode.

2. In a spark plug, a generally tubular metal shell having an internal annular groove adjacent the base there of, said base including an inwardly extending annular flange which forms the bottom wall of said groove a metal ground electrode having an annular portion secured within said groove thereby forming a permanent joint between said electrode and the base of said shell, anda thin layer of heat and corrosion resistant alloy containing about from to nickel, from 5% to 15% phos phorous and up to 5% boron bonded to said baseand to said electrode, said layer forming a protective coating on said base and over the joint between said base and said electrode.

3. A spark plug comprising a generally tubular metal shell having an internal annular groove adjacent the base thereof, said base including an inwardly extending annular flange which forms the bottom wall of said groove, a metal ground electrode having an annular portion welded within said groove thereby forming a permanent joint between said electrode and the base of saidshell, and a thin layer of heat and corrosion resistantnickel base alloy bonded to said base and to said electrode, said layer forming a protective coating on said base and over the joint between said base and said electrode.

4. A spark plug as set forth in claim 3 wherein the nickel base alloy contains from 80% to 95% nickel, from 5% to 15% phosphorous, from .5% to 5% boron plus small amounts of silicon and manganese. I i

5. In a method for manufacturing a spark plug, the improvement which comprises the steps of welding a ground electrode within a groove'in the base of a generally tubular metal shell and then coating 'saidlelectrode and the base portion of said shell with a thin layer of a nickelbase alloy to thereby form a heat and corrosion resistant covering over the welded connection between said shell and said electrode.

6. A method for manfacturing a spark plug, including the steps of forming a tubular metal shell having an opening in the form of a counterbore at the lower end thereof, positioning a ground electrode having an annular portion concentrically within said opening, welding the anular portion of said ground electrode to said shell, turning the lower circumferential edge of said shell inwardly over said annular portion, applying a nickel base alloy brazing paste over the junction between said shell and said electrode and over said inwardly turned flange, and then applying heat to bond said nickel base alloy to said flange and said electrode and form a heat and corrosion resistant protective coating.

7. In a method for manufacturing a spark plug of the type having a metal ground electrode with an annular portion secured concentrically to the base of a tubular metal shell, the improvement which comprises the steps of welding the annular portion of the ground electrode to the base of the tubular metal shell, applying a nickel base alloy brazing material over the base of said shell and the joint between said electrode and said shell, and then applying heat to cause said brazing material to melt and bond to said shell and said electrode to form a heat and corrosion resistant protective coating.

8. A method for manufacturing a spark plug, including the steps for forming a tubular metal shell having an opening in the form of a counterbore at the lower end thereof, forming a ground electrode with an annular portion sized to fit snugly within said counterbore, positioning said ground electrode concentrically within said opening, welding the annular portion of said ground electrode to said shell, turning the lower circumferential edge of said shell inwardly over said annular portion, applying a nickel base alloy brazing material over the junction of said shell and said electrode and over said inwardly turned flange, and then heating said material to a temperature of about 1600" F. to 1900 F. to bond the nickel base alloy to said flange and said electrode and form a heat and corrosion resistant protective coating.

9. A method for manufacturing a spark plug, including the steps for forming a tubular metal shell having an opening in the form of a counterbore at the lower end thereof, forming a ground electrode with an annular portion sized to fit snugly within said counterbore, positioning said ground electrode concentrically Within said opening, welding the annular portion of said ground electrode to said shell, turning the lower circumferential edge of said shell inwardly over said annular portion, applying a nickel base alloy brazing material over the junction of said shell and said electrode and over said inwardly turned flange, and then heating said material to a temperature of about 1600 F. to about 1900 F. to bond the nickel base alloy to said flange and said electrode and form a heat and corrosion resistant protective coating, said brazing rnaterial containing about 38 parts by weight flux and 62 parts by weight of an alloy containing from to nickel, from 5% to 15% phosphorous, from .5 to 4% boron plus small amounts of silicon manganese.

References Cited in the file of this patent UNITED STATES PATENTS 1,369,386 Brewer Feb. 22, 1921 2,164,311 Doran Oct. 4, 1932 2,350,731 Dobrosavljevitch June 6, 1944 2,377,481 Christie June 5, 1945 2,394,865 McCarthy Feb. 12, 1946 2,684,665 Tognola June 9, 1949 2,743,177 Cape Apr. 24, 1956 2,755,183 Cape a- July 17, 1956 2,762,706 Cape Sept. 11, 1956 2,840,628 Candelise June 24, 1958 FOREIGN PATENTS 117,537 Great Britain July 25, 1918 487,263 Great Britain June 17, 1938 

1. IN A SPARK PLUG, A GENERALLY TUBULAR METAL SHELL HAVING AN ANNULAR BASE, A METAL GROUDN ELECTRODE HAVING A PORTION THEREOF WELDED WITHIN A GROOVE IN THE BASE OF SAID SHELL THEREBY FORMING A PERMANENT JOINT BETWEEN SAID ELECTRODE AND SAID BASE, AND A THIN LAYER OF HEAT AND CORROSION RESISTANT NICKEL BASE ALLOY BONDED TO SAID 